Thermally-Protected Scintillation Detector

ABSTRACT

Systems, methods, and devices for thermally protecting a scintillator crystal of a scintillation detector are provided. In one example, a thermally-protected scintillator may include a scintillator crystal and a thermal protection element, which may partially surround the scintillator crystal. The thermal protection element may be configured to prevent the scintillator crystal from experiencing a rate of change in temperature sufficient to cause cracking or non-uniform light output, or a combination thereof.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/573,194, filed Oct. 5, 2009, which claims benefit of U.S.Provisional Application No. 61/104115, filed on Oct. 9, 2008; U.S.Provisional Application No. 61/160734, filed on Mar. 17, 2009; and U.S.Provisional Application No. 61/160746, filed Mar. 17, 2009. Each of theaforementioned related patent applications is herein incorporated byreference.

BACKGROUND

The present disclosure relates generally to scintillation detectors and,more particularly, to thermal protection for scintillation detectors.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present disclosure,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

Scintillation detectors are employed in a variety of settings to detectnuclear and electromagnetic radiation. In the presence of suchradiation, a scintillator crystal may produce detectable wavelengths oflight. This light may be converted to an electrical signal by a lightdetection device, such as a photomultiplier tube, and the electricalsignal may be subsequently analyzed to determine, for example, an amountof detected radiation. By way of example, scintillation detectors mayassist in the indirect determination of formation lithology by detectinggamma-ray scattering in a subterranean formation, as well as the directdetermination of the formation lithology by detecting neutron-inducedgamma-rays caused by neutrons emitted into the subterranean formation.

When scintillation detectors are employed for downhole well-logging, thescintillator crystals of such scintillation detectors may be subjectedto a rapid increase or decrease in temperature due to heat from thesurrounding formation. Certain scintillation detectors, such as NaI(T1)detectors, may operate correctly at temperatures up to 200° C. withoutany protection. Many new scintillation materials, such as LaBr₃:Ce andLaCl₃:C, among others, may function at temperatures even beyond 200° C.Many of the new scintillation materials, however, while capable ofoperating at a very high temperature, may tend to crack or shatter ifheated or cooled too rapidly.

SUMMARY

Certain aspects commensurate in scope with the originally claimedembodiments are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms the embodiments might take and that these aspects arenot intended to limit the scope of the presently disclosed subjectmatter. Indeed, the embodiments may encompass a variety of aspects thatmay not be set forth below.

Embodiments of the present disclosure relate to systems, methods, anddevices for thermally protecting a scintillator crystal of ascintillation detector. In one example, a thermally-protectedscintillator may include a scintillator crystal and a thermal protectionelement, which may partially surround the scintillator crystal. Thethermal protection element may be configured to prevent the scintillatorcrystal from experiencing a rate of change in temperature sufficient tocause cracking and/or non-uniform light output, or a combinationthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the presently disclosed subject matter may become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a schematic block diagram of a well logging system employing athermally protected scintillation detector, in accordance with anembodiment;

FIG. 2 is a schematic cross-sectional view of a thermally-protectedscintillation detector employing a hygroscopic scintillator crystal, inaccordance with an embodiment;

FIG. 3 is a schematic cross-sectional view of a thermally-protectedscintillation detector employing a non-hygroscopic scintillator crystal,in accordance with an embodiment;

FIG. 4 is a schematic cross-sectional view of a thermally-protectedscintillation detector in which a shock absorption layer and thermallyconductive layer are combined, in accordance with an embodiment;

FIG. 5 is a schematic cross-sectional view of a thermally-protectedscintillation detector having a thermal protection element extendingover a photomultiplier tube, in accordance with an embodiment;

FIG. 6 is a schematic cross-sectional view of a thermally-protectedscintillation detector having a thermal protection element partiallyextending over a photomultiplier tube, in accordance with an embodiment;

FIG. 7 is a schematic cross-sectional view of a thermally-protectedscintillation detector employing a partially-open Dewar flask integratedwith a housing of a scintillator crystal, in accordance with anembodiment;

FIG. 8 is a schematic cross-sectional view of a thermally-protectedscintillation detector employing an extended partially-open Dewar flaskwith magnetic shielding, in accordance with an embodiment;

FIG. 9 is a schematic cross-sectional view of a thermally-protectedscintillation detector employing an extended partially-open Dewar flaskintegrated with a housing of a scintillator crystal, in accordance withan embodiment;

FIG. 10 is a perspective view of a high temperature heater layerintegrated into a thermally-protected scintillation detector, inaccordance with an embodiment;

FIG. 11 is a schematic cross-sectional view of a scintillation detectoremploying the heater of FIG. 10, in accordance with an embodiment;

FIG. 12 is a flowchart describing an embodiment of a method forperforming a warm-up procedure using the thermally-protectedscintillation detector of FIG. 11; and

FIG. 13 is a flow chart describing an embodiment of a method forperforming a cool-down procedure using the thermally-protectedscintillation detector of FIG. 11.

DETAILED DESCRIPTION

One or more specific embodiments of the present disclosure will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

Present embodiments relate generally to scintillation detectors for usein high-temperature environments (e.g., approximately 200° C. andabove), which may include subterranean wells. Specifically, becauseentering and exiting such high-temperature environments may causescintillator crystals capable of high-temperature operation to rapidlyheat up and rapidly cool down, unprotected scintillator crystals maycrack or break due to temperature stresses. Accordingly, various passiveand active thermal-protection schemes for scintillator crystals areprovided below. These thermal protection schemes may be simpler,smaller, and less expensive than traditional complete Dewar flasks,which may use a complex and mechanically fragile design and whichtypically may include a large thermal mass with a long stopper to allowextended use of the scintillation detector before exceeding its designtemperature.

As described in the present disclosure, one or more thermal protectionelements integrated into the package of the scintillator may reduce therate of temperature change experienced by the scintillation detector,which may limit internal thermal stresses and may reduce the likelihoodof scintillator damage caused by such stresses. Additionally oralternatively, embodiments of scintillator packages incorporating thesethermal protection elements may provide an even temperature distributionin the scintillator crystal. This even temperature distribution mayimprove the spectroscopy performance of the scintillator crystal, as thelight output of the scintillator crystal may be a function of the localtemperature of the scintillator material. If the light output of thescintillator crystal is not uniform throughout, the energy resolutionand associated spectroscopy performance may be impaired. In thefollowing disclosure, the thermal protection elements may both reducethermal stress and provide greater uniformity of light output byreducing thermal gradients in the scintillator crystal material.

In one example of a passive thermal protection element incorporated intoa scintillator package, one or more thermal insulation layers maypartially surround the scintillator crystal. These thermal insulationlayers may reduce the rate of heat transfer to and from the scintillatorcrystal. Similarly, one or more thermally conductive layers maypartially surround the scintillator crystal to provide for more evenheating and cooling of the scintillator crystal. In another example, apartially-open Dewar flask may partially surround the scintillatorcrystal. These thermal protection elements may be extended beyond thescintillator component to partially surround a photomultiplier tube(PMT) component of the scintillation detector, which may further enhancethe thermal protection of the scintillator crystal of the scintillationdetector. In addition, certain embodiments of the partially-open Dewarflask may be modified to include magnetic shielding as well.

As noted above, a thermal protection element incorporated into ascintillator package may also actively prevent temperature stresses fromharming the scintillator crystal. The active measures may include, forexample, partially surrounding the scintillator crystal with a heatingdevice. Prior to placing the scintillation detector into ahigh-temperature environment, the heating device may heat thescintillator crystal such that the rate of temperature change does notexceed a threshold amount (e.g., 2° C. per minute). Similarly, when thescintillation detector is removed from the high-temperature environment,the heating device may occasionally heat the scintillator crystal as thecrystal cools to prevent the temperature from changing at a rate thatwould exceed the threshold amount. In this way, temperature stressesfrom rapid temperature change may be actively averted. These activemeasures may be combined with the passive measures discussed above.

With the foregoing in mind, FIG. 1 represents an embodiment of awell-logging system 10 employing a thermally-protected scintillationdetector. As illustrated in FIG. 1, the well-logging system 10 mayinclude a downhole tool 12 and data processing circuitry 14. By way ofexample, the downhole tool 12 may be a slickline or wireline tool forlogging an existing well, or may be installed in a borehole assembly forlogging while drilling (LWD). The data processing system 14 may beincorporated into the downhole tool 12 or may be at a remote location.The downhole tool 12 may include an external housing 16 that includes avariety of well-logging components.

In some embodiments, a radiation source 18 may be employed to emitradiation into a surrounding formation, which may emit such radiation asneutrons, gamma rays, and/or other particles or electromagneticradiation. By way of example, the radiation source 18 may be anelectronic neutron source, such as a Minitron™ by SchlumbergerTechnology Corporation by Schlumberger Technology Corporation. In otherembodiments, the downhole tool 12 may not include the radiation source18.

If a radiation source 18 is employed, a shield 20 may prevent errantradiation from traveling directly to a thermally-protected scintillationdetector 22. The scintillation detector 22 may include a scintillatorcomponent 24 and a photomultiplier tube component 26. A scintillatorcrystal in the scintillator component 24 may generate light in thepresence of certain radiation (e.g., x-rays or gamma-rays), which mayoccur spontaneously in the surrounding formation or when radiation fromthe radiation source 18 interacts with the surrounding formation. Thephotomultiplier tube component 26 may generate an electrical signal fromthe light generated by the scintillator crystal in the scintillatorcomponent 24. As described below, a package encompassing thescintillator component 24 may incorporate passive and/or active thermalprotection elements for the scintillator crystal of the scintillatorcomponent 24 that partially surround the scintillator crystal. In someembodiments, the thermal protection element and/or the package for thescintillator crystal of the scintillator component 24 may extend overall or a portion of the photomultiplier tube component 26.

As discussed below, the scintillator component 24 may include ahigh-temperature scintillator crystal. Such a high-temperaturescintillator crystal may be hygroscopic or non-hygroscopic. Examples ofhygroscopic high-temperature scintillator crystals may includescintillator crystals of LaBr₃:Ce and/or LaCl₃:C available fromSaint-Gobain, as well as scintillator crystals of mixed La-halidesavailable from General Electric Company. Various oxide-basedscintillator crystals with excellent high temperature performance mayalso be used, many of which are non-hygroscopic. These may include, forexample, LuAP:Ce, LuYAP:Ce, YAP:Ce, LuAG:Pr, and LPS (LutetiumPyro-Silicate, Lu₂Si₂O₇), to name a few.

The high-temperature scintillator crystals mentioned above may becapable of operating at temperatures much greater than 200° C. However,these scintillator crystals may crack or break if subjected to rapidtemperature changes. Thus, as described in greater detail below, thethermally-protected scintillation detectors 24 may include passiveand/or active measures to prevent such rapid temperature changes. Thepassive measures may include thermal insulation or a partially-openDewar flask to reduce heat transfer from the surrounding formation intothe scintillator crystal. The active measures may include controllingthe rate of temperature increase of the scintillator crystal by heatingthe scintillator crystal before and after the scintillator detectorenters a high-temperature environment.

Signals from the thermally-protected scintillation detectors 22 may betransmitted to the data processing system 14 as data 28. The dataprocessing system 14 may include a general-purpose computer, such as apersonal computer, configured to run a variety of software, includingsoftware implementing a technique for determining formation propertiesbased on radiation detected by the scintillation detectors 22. In someembodiments, the data processing system 14 may be an embedded processorin the downhole tool 12.

The downhole tool 12 may transmit the data 28 to the data acquisitioncircuitry 30 of the data processing system 14 via, for example, atelemetry system communication downlink or a communication cable. Afterreceiving the data 28, the data acquisition circuitry 30 may transmitthe data 28 to data processing circuitry 32. In accordance with one ormore stored routines, the data processing circuitry 32 may process thedata 28 to ascertain one or more properties of a subterranean formationsurrounding the downhole tool 12, which may be indicated by a report 34.The data processing circuitry 32 may employ any suitable technique fordetermining the properties of the subterranean formation.

FIGS. 2-9 represent embodiments of the scintillation detectors 22incorporating passive measures to protect high-temperature scintillatorcrystals. Specifically, FIGS. 2-6 represent embodiments employingthermal insulation and FIGS. 7-9 represent embodiments employing apartially-open Dewar flask, which can also be considered thermalinsulation, but due to its characteristics, may impart certainadditional benefits. Turning first to FIG. 2, the thermally-protectedscintillation detector 22 may include the scintillator component 24 andthe photomultiplier component 26. The scintillator component 24 mayinclude a scintillator crystal 36, a reflector layer 38, a shockabsorber layer 40, a thermal conductor layer 42, a thermal insulationlayer 44, and a scintillator housing 46. In general, the various layerssurrounding or partially surrounding the scintillator crystal 36 mayform the “package” which encapsulates or partially encapsulates thescintillator crystal 36. In the embodiment of FIG. 2, the scintillatorcrystal 36 may be hermetically sealed behind an optical coupling 48 anda window 50 to prevent moisture from damaging the scintillator crystal36. As such, the scintillator crystal 36 may be a hygroscopic ornon-hygroscopic scintillator crystal capable of operating under hightemperatures. Such scintillator crystals may include, for example,LaBr₃:Ce or LaCl₃:Ce scintillator crystals available from Saint-Gobain,scintillator crystals of mixed La-halides available from GeneralElectric Company, and/or oxide-based scintillator crystals withexcellent high temperature performance such as LuAP:Ce, LuYAP:Ce,YAP:Ce, LuAG:Pr, and LPS (Lutetium Pyro-Silicate, Lu₂Si₂O₇).

The reflector layer 38 may surround the scintillator crystal 36 toreflect light generated by the scintillator crystal 36 toward thephotomultiplier component 26. The reflector layer 38 may include, forexample, Teflon®, Al₂O₃ or TiO₂, or other materials in the form ofsheets, cast shapes, powders or paint. The next layer surrounding thereflective layer 38 may be the shock absorber layer 40. The shockabsorber layer 40 may be capable of contracting or expanding toaccommodate differential thermal expansion and/or contraction of thescintillator crystal 36. The shock absorber material 40 may be a solidmaterial like a high temperature elastomer (e.g. Viton or a Siliconebased Elastomer) and/or may include radial or axial springs. Theelastomer may include ribs or other features to provide room for itsthermal expansion or contraction while providing mechanical support tothe scintillator 36.

The thermal conductor layer 42 and the thermal insulation layer 44 mayoperate in concert to reduce the rate of temperature change in anyparticular location of the scintillator crystal 36. In particular, thethermal insulation layer 44 may reduce the rate at which heat istransferred between the scintillator housing 46 and the remaining layersbetween the scintillator housing 46 and the thermal conductor 42. Thethermal insulation layer 44 may include, for example, various elastomersand similar materials (e.g., a viton sheet or silicone), fiberglass, anaerogel, plastics (e.g., peek), Teflon® materials such asperfluoroalkoxy polymer resin (PFA), polytetrafluoroethylene (PTFE), orfluorinated ethylene propylene (FEP), and/or a polyimide film such asKapton®. As illustrated in FIG. 2, the thermal insulation layer maypartially surround the scintillator crystal 36, leaving an opening forthe optical coupling 48 and window 50. The thermal conductor layer 42may distribute any heat transferred through the thermal insulation layer44 evenly across the surface of the scintillator 36. In certainembodiments, the thermal conductor layer 42 may cover greaterscintillator crystal 36 surface by extending into the center of thescintillator crystal 36 via a hole drilled in the scintillator 36. Thethermal conductor layer 42 may include any thermally-conductivematerial, such as aluminum, copper, or stainless-steel.

The scintillator housing 46 may represent any standard housing for ascintillator crystal. In some embodiments, an additional thermalconductor layer may surround all or part of the scintillator housing 46to insure heat is evenly distributed across the surface area of thescintillator housing 46. Since the scintillator crystal 36 may be ahygroscopic scintillator crystal, the scintillator housing 46 may beconstructed to seal the scintillator crystal 36 from external moisture.As such, an optical coupling 48 may join the scintillator crystal 36 toan optical window 50 attached to the scintillator housing 46.

The photomultiplier component 26 may similarly include an opticalcoupling 48 and a window 50 to connect to the scintillator component 24.The photomultiplier component may include a photomultiplier tube 52,which may not necessarily include thermal protection, as many availablephotomultiplier tubes 52 may be capable of operating under rapidlyvarying temperatures. In alternative embodiments, the photomultipliercomponent 26 may include a micro-channel plate (MCP) in lieu of thestandard multiplier structure or the photomultiplier component 26 may bean avalanche photodiode (APD). Additionally, while the photomultipliercomponent 26 is illustrated as optically coupled to the scintillatorcomponent 24, which is hermetically sealed to protect the scintillatorcrystal 36, it also may be possible to mount the scintillator crystal 36directly to the photomultiplier tube 52, if both are encased in a singlehermetically sealed package.

FIG. 3 represents an alternative embodiment of the scintillationdetector 22 illustrated in FIG. 2. The embodiment of the scintillationdetector 22 of FIG. 3 is substantially the same as that of FIG. 2,including the scintillator component 24 and the photomultipliercomponent 26. The scintillator component 24 may similarly include thescintillator crystal 36, the reflector layer 38, the shock absorberlayer 40, the thermal conductor layer 42, the thermal insulation layer44, and the scintillator housing 46. However, the scintillation crystal36 may be a non-hygroscopic scintillation crystal, such as anoxide-based scintillator crystal with high temperature capabilities,such as LuAP:Ce, LuYAP:Ce, YAP:Ce, LuAG:Pr, or LPS (LutetiumPyro-Silicate, Lu₂Si₂O₇). Because the scintillator crystal 36 isnon-hygroscopic, the thermal protection elements need not be sealedwithin in the scintillator housing 46. As such, the optical coupling 48and window 50 may be omitted from the scintillator component 24. Likethe embodiment discussed above, the optical coupling 48 and the opticalwindow 50 of the photomultiplier component 26 may optically couple thescintillator crystal 36 to the photomultiplier tube 52.

FIG. 4 represents another embodiment of the scintillation detector 22,in which the shock absorbing layer 40 may be combined into the thermalinsulation layer 44. Like the embodiment of the scintillation detector24 of FIG. 2, the embodiment of the scintillation detector 22 of FIG. 4may include the scintillator component 24 and the photomultipliercomponent 26. The scintillator component 24 may similarly include thescintillator crystal 36, the reflector layer 38, the thermal conductorlayer 42, the thermal insulation layer 44, and the scintillator housing46. Like the embodiments discussed above, the optical couplings 48 andthe optical windows 50 may optically couple the scintillator crystal 36to the photomultiplier tube 52. However, the thermal insulation layer 44may be designed to incorporate the shock absorbing characteristicsassociated with the shock absorbing layer 40 of the embodiments of FIGS.2 and 3 above. The material could be a high temperature elastomer (e.g.Viton or a high temperature Silicone elastomer). In some embodiments,when the reflector layer 38 employs a metallic reflector, such assilver, the reflector layer 38 may also be combined into the thermalconductor layer 42.

It should be noted that, in some embodiments, the thermal conductorlayer 42 may be eliminated entirely if the thermal protection providedby the thermal insulation layer 44 is sufficient. In other words, if thethermal conductivity of the scintillator crystal 36 material and thereduced rate of temperature increase provided by the thermal insulationlayer 44 are sufficient to protect the scintillation crystal 36 fromexcessive thermal stress, the thermal conductor layer 42 may also beomitted.

For certain applications of the downhole tool 12, such as traversing azone of steam flood during well logging, the temperature of thesurrounding environment may increase very rapidly. Under such extremeconditions, the thermal protection measures discussed above may notsufficiently protect all elements of the scintillation detector 22. Toprotect the photomultiplier tube 52 from these high temperatures, aswell as to prevent the transfer of heat through the window 50 via thephotomultiplier tube 52, the thermal protection measures described abovemay be adapted. In particular, these adaptations may take two forms,including mounting the entire scintillation detector 22 inside athermally protective housing and expanding the thermal protectionsdescribed above to cover all or part of the photomultiplier tube 52. Inthe first case, the thermal protection measures may not be integratedinto the housing of the scintillator component 24, but the outerdiameter of the scintillation detector 22 may be expanded. To maintainthe same diameter, the size of the scintillator crystal 36 may bereduced. The second case, in which the thermally protective elementshave been expanded to cover all or part of the photomultiplier tube 52,is illustrated in FIG. 5. In general, the thermal protection may notextend over any heat-generating elements, such as resistors, diodes oractive electronic components that may be mounted on the photomultipliertube 52 to provide the correct operating voltage or to amplify thesignals available at the output of the photomultiplier. In addition,extending the thermal protection beyond the photomultiplier window helpsinsure a more uniform temperature distribution on the photocathode andthereby a better spectroscopy performance.

The embodiment of the scintillation detector 22 of FIG. 5 may includethe scintillator component 24 and the photomultiplier component 26. Thescintillator component 24 may include the scintillator crystal 36, thereflector layer 38, the shock absorbing layer 40, the thermal conductorlayer 42, the thermal insulation layer 44, and the scintillator housing46. Like the embodiments discussed above, the optical couplings 48 andthe optical windows 50 may optically couple the scintillator crystal 36to the photomultiplier tube 52. In the embodiment of FIG. 5, however,the thermal insulation layer 44 and the thermal conductor layer 42 mayextend over the photomultiplier tube 52, surrounded by an outer housing46. This may reduce the amount of heat that may reach the scintillatorcrystal 36 via the photomultiplier tube 52. The thermal conductor layer42 should not extend over the photomultiplier tube 52 to preventexcessive heat from reaching the scintillator crystal 36 through thethermal conductor layer 42.

As shown in FIG. 6, the thermal insulation layer 44 may extend only asfar as may optimally provide a reduction in heat transfer to thescintillation crystal 36 from an external high-temperature environment.In FIG. 6, the scintillator component 24 of the scintillation detector22 may include the scintillator crystal 36, the reflector layer 38, theshock absorbing layer 40, the thermal conductor layer 42, the thermalinsulation layer 44, and the scintillator housing 46. Like theembodiments discussed above, the optical couplings 48 and the opticalwindows 50 may optically couple the scintillator crystal 36 to thephotomultiplier tube 52 of the photomultiplier component 26.

Unlike the embodiments described above, the embodiment of thescintillation detector 22 of FIG. 6 may include partial thermalprotection over the photomultiplier tube 52. The non-thermally protectedlength of the photomultiplier tube 52 is denoted as L₁, while thethermally-protected length is denoted as L₂. Extending the length L₂beyond a certain distance may provide diminishing thermal protection forthe scintillator crystal 36, but may add additional manufacturing costs,weight, and size to the scintillation detector 22. Accordingly, optimaldistances L₁ and L₂ may be determined by modeling the reduction in heattransfer to the scintillator crystal 36 at various values of L₁ and/orL₂.

FIGS. 7-9 represent embodiments of the thermally-protected scintillationdetector 22 employing a partially-open Dewar flask to obtain thermalprotection from rapid heating and cooling. In the embodiment of FIG. 7,the scintillation detector 22 may include the scintillator component 24and the photomultiplier component 26. The scintillator component 24 mayinclude the scintillator crystal 36, the reflector layer 38, the shockabsorber layer 40, the thermal conductor layer 42, and the scintillatorhousing 46. Like the embodiments discussed above, the optical couplings48 and the optical windows 50 may optically couple the scintillatorcrystal 36 to the photomultiplier tube 52. The scintillator housing 46may be constructed to form a partially-open Dewar flask that causes avacuum 54 to separate the two housing layers 46. Following constructionof the partially-open Dewar flask, the two housing layers 46 may includea pinch-off 56 and weld 58. It should be understood that certain detailsregarding the construction of the partially-open Dewar flask, such asinternal supports, thermal radiation reflectors, and so forth, are notshown, as they are well known in the art.

Like the embodiments discussed above, certain thermally-protectiveelements may extend to cover all or part of the photomultiplier tube 52,which may also serve to thermally protect the scintillator crystal 36 ina manner similar to a stopper in a traditional Dewar flask. In theembodiment illustrated in FIG. 7, a layer of electrical and/or thermalinsulation 60 may shield a portion of the photomultiplier tube 52beneath the outer layer of the scintillator housing 46. The precisedistance over which the layer 60 and scintillator housing 46 may extendmay be determined through thermal and/or electrical modeling. The layer60 may cover few, if any, heat-generating components of thephotomultipler tube 52, such as resistors, diodes, or active electroniccomponents that may be mounted on the photomultiplier tube 52.

FIG. 8 illustrates an alternative embodiment of the scintillationdetector 22 illustrated in FIG. 7, in which the partially-open Dewarflask may be completely separate from the scintillator housing 46. Inthe embodiment of FIG. 8, the scintillation detector 22 may include thescintillator component 24 and the photomultiplier component 26. Thescintillator component 24 may include the scintillator crystal 36, thereflector layer 38, the shock absorber layer 40, the thermal conductorlayer 42, and the scintillator housing 46. Like the embodimentsdiscussed above, the optical couplings 48 and the optical windows 50 mayoptically couple the scintillator crystal 36 to the photomultiplier tube52. The partially-open Dewar flask may be formed by an inner wall 62 andan outer wall 64 joined by a weld 58, and the space between may beevacuated to produce an insulative vacuum 54. Like the embodiment ofFIG. 7, a pinch-off 56 may be used in forming the partially-open Dewarflask. In some embodiments, it may not be practical or desirable toextend the partially-open Dewar flask over the photomultiplier tube 52.Under such conditions, the inner wall 62 and outer wall 64 may beshortened to approximately the length of the partially-open Dewar flaskof FIG. 7. The partially-open Dewar flask may cover few, if any,heat-generating components of the photomultipler tube 52, such asresistors, diodes, or active electronic components that may be mountedon the photomultiplier tube 52.

Because the partially-open Dewar flask is constructed in such a way asto overlap the front end of the photomultiplier tube 52, thermal leakagefrom the photomultiplier 52 may be reduced, which may assure a moreuniform scintillator crystal 36 temperature. Indeed, the photomultiplier52 may effectively thermally protect the scintillator crystal 36 in amanner similar to a stopper in a traditional Dewar flask. Additionally,the shape of the partially-open Dewar flask may also result in a uniformtemperature of a photocathode of the photomultiplier tube 52. Anon-uniform photocathode temperature may lead to a non-uniform specialdistribution of the quantum efficiency (QE) of the photomultiplier tube52 and, as a consequence, may lead to poorer spectroscopy performance.

Using the embodiment of FIG. 8, the thermal protection function providedby the partially-open Dewar flask may be combined with a magneticshielding function. Specifically, the materials of the inner wall 62 andthe outer wall 64 of the partially-open Dewar flask may be chosen tomagnetically shield the photomultiplier tube 52. For example, the innerwall 62, the outer wall 64, or both the inner wall 62 and the outer wall64 may be constructed of materials with high magnetic permeability.Additionally or alternatively, the inner wall 62 may have a layer of amaterial with a very high permeability and a relatively low saturation(e.g., Admu 80 from AD-Vance Magnetics), and the outer wall 64 may beconstructed of or may have a layer of a material with a lowerpermeability and higher saturation (e.g., soft iron).

FIG. 9 illustrates another alternative embodiment of the scintillationdetector 22 of FIG. 7, in which the partially-open Dewar flask may beformed in the scintillator housing 46, but which may extend to cover allor part of the photomultiplier tube 52. In the embodiment of FIG. 9, thescintillation detector 22 may include the scintillator component 24 andthe photomultiplier component 26. The scintillator component 24 mayinclude the scintillator crystal 36, the reflector layer 38, the shockabsorber layer 40, the thermal conductor layer 42, and the scintillatorhousing 46. Like the embodiments discussed above, the optical couplings48 and the optical windows 50 may optically couple the scintillatorcrystal 36 to the photomultiplier tube 52. The two scintillator housinglayers 46 may extend to cover all or part of the photomultiplier tube52, between which a partially-open Dewar flask may be formed. As such,the scintillation detector 22 may also include the weld 58 and thepinch-off 56. Some embodiments may also include the layer of electricaland/or thermal insulation 60. To effectively thermally insulate thescintillator crystal 36, the layer 60 may cover few, if any,heat-generating components of the photomultipler tube 52, such asresistors, diodes, or active electronic components that may be mountedon the photomultiplier tube 52.

The scintillator housing 46 and optical windows 50 in the embodiments ofFIGS. 7-9 are illustrated as hermetically sealing the scintillatorcrystal 36. However, if the scintillator crystal 36 is non-hygroscopic,the scintillator crystal 36 may alternatively couple directly to thewindow 50 of the photomultiplier tube 52, as generally illustrated abovewith reference to FIG. 3. Moreover, even if the scintillator crystal 36is hygroscopic, the scintillator crystal 36 may couple directly to thewindow 50 of the photomultiplier tube 52 if the photomultiplier tube 52is hermetically sealed with the scintillator crystal 36.

FIGS. 10-13 describe a manner of actively providing thermal protectionfor the scintillator crystal 36 of the thermally-protected scintillationdetector 22. Because certain scintillator crystals (e.g., LaBr₃) maycrack or break if the rate of temperature change exceeds a thresholdrate of change (e.g., 2° C. per minute), FIG. 10 illustrates a heatingdevice 66 that may be used to prevent the scintillator crystal 36 fromheating or cooling too quickly. Specifically, the heating device 66 maypreheat the scintillator crystal 36 before the scintillator crystal 36enters a high-temperature environment to prevent excessive temperatureincreases, and may occasionally provide heat after the scintillatorcrystal 36 exits the high-temperature environment to prevent excessivetemperature decreases. The heating device 66 may be, for example, apolyimide heater pad or sleeve, such as the Kapton® heater by Hi-HeatIndustries.

As illustrated in FIG. 10, the heating device 66 may receive electricalpower via electrical leads 68. The electrical power may travel through aresistive path 70 to generate heat. In some embodiments, the heatingdevice 66 may be a flexible film etched onto a metal foil, such aspolyimide. Such a heating device 66 may withstand extreme temperatureranges, including high temperatures (e.g., 200° C. or greater). Theheating device 66 may have rapid warm-up times and a quick response, asthe resistive path 70 may run cooler. As such, the heating device 66 maythus be ideal for service in harsh environments such as subterraneanformations.

The heating device 66 may be relatively thin, having a thickness D ofapproximately 0.005 inches, and may include a control circuit, as wellas temperature sensors and other conventional devices for heaters. Usingthe temperature sensors, the control circuit may carry out suchalgorithms as described below with reference to FIGS. 12 and 13 forwarming and cooling a scintillator crystal 36 to prevent excessivetemperature change. Additionally or alternatively, the data processingsystem 14 may control the heating device 66, in which case the dataprocessing system 14 may carry out these algorithms. It should be notedthat the heating device 66 may have significant power density; in oneembodiment, the heating device 66 may have a density of 5 watts persquare inch, at 120V.

To heat the scintillator crystal 36 of the scintillation detector 22,the heating device 66 may be mounted to a portion of the outer housing46 or installed internally to the housing 46. Generally, if the heatingdevice 66 is mounted to the outer housing 46, other thermal protectionshould not be employed by the scintillation detector 22. Additionally oralternatively, the heating device 66 may be disposed internally to ascintillation detector 22 that employs passive thermal protectionmeasures. One such configuration is illustrated in FIG. 11.

FIG. 11 represents one embodiment of the thermally-protectedscintillation detector 22 that employs the heating device 66, withcertain control algorithms, to prevent rapid heating and/or cooling ofthe scintillator crystal 36. Although the scintillation detector 22 ofFIG. 11 is illustrated as including passive measures (e.g., the thermalinsulation layer 44 and/or the thermal conductor layer 42), the heatingdevice 66 may be employed with or without such passive thermalprotection. In the embodiment of FIG. 11, the scintillation detector 22may include the scintillator component 24 and the photomultipliercomponent 26. The scintillator component 24 may include the scintillatorcrystal 36, the reflector layer 38, the shock absorber layer 40, thethermal conductor layer 42, the thermal insulation layer 44, and thescintillator housing 46. Like the embodiments discussed above, theoptical couplings 48 and the optical windows 50 may optically couple thescintillator crystal 36 to the photomultiplier tube 52.

Thermally coupled to the thermal conductor layer 42, the heating device66 may cause heat to evenly reach the scintillator crystal 36 to preventcracking or breaking A temperature sensor 72, if not integrated into theheating device 66, may measure the surface temperature of thescintillator crystal 36. A control circuit associated with the heatingdevice 66, or the data processing circuitry 14, may control when theheating device 66 is active based on temperatures detected by thetemperature sensor 72.

FIG. 12 is a flowchart 74 describing an embodiment of a method forperforming a warm-up procedure using the heating device 66, which mayslowly heat the scintillator crystal 36 prior to its introduction to ahigh-temperature environment, such as a subterranean well. The methodprovided by flowchart 74 of FIG. 12 may be implemented by controlcircuitry included in the heating device 66 or by the data processingcircuitry 14. In a first step 76, the control circuitry may begin thewarm-up procedure because the scintillation detector 22 is to be placedin a high-temperature environment. In step 78, the heating device 66 maybe activated, causing the scintillator crystal 66 to be heated.

As indicated by decision block 80, if the temperature of thescintillator crystal 86 has reached a target temperature (e.g., theexpected temperature of a downhole formation), the heating device 66 maybe deactivated in step 82. The scintillator detector 22 may thereafterenter the high-temperature environment without experiencing a rapidincrease in the scintillator crystal 36 temperature, which may cause thescintillator crystal 36 to become damaged. If the target temperature hasnot been reached, the process may flow to decision block 84.

In decision block 84, the control circuitry may consider whether thetemperature increase has approached and/or exceeded a threshold rate ofchange. The particular threshold rate of change may vary depending onthe characteristics of the scintillator crystal 36. By way of example,if the scintillator crystal 36 is formed of LaBr₃, the designatedmaximum rate of temperature change may be 2° C. per minute. If the rateof temperature increase does not exceed the threshold rate of change,the heating device 66 may continue to heat the scintillator crystal 36until the target temperature is reached, as shown in decision block 80,or until the rate of change approaches the threshold, as shown indecision block 84. On the other hand, if the rate of temperature changedoes approach the threshold rate of change, the amount of power suppliedto the heating device 66 may be decreased and/or the heating device 66may be briefly deactivated, in step 86. When the heating device 66becomes active again, the process may continue until the scintillatorcrystal 36 reaches the target temperature.

FIG. 13 describes an embodiment of a method for regulating a cool-downof the scintillation detector 22 using the heating device 66, which mayensure that the scintillator crystal 36 does not too rapidly cool afterexiting a high-temperature environment, such as a subterranean well. Themethod provided by the flowchart 88 of FIG. 13 may be implemented bycontrol circuitry included in the heating device 66 or by the dataprocessing circuitry 14. The control circuitry may continuously monitorthe cool down rate in step 90 by comparing the current temperature withprevious temperatures. In decision block 92, the control circuitry maycheck the cool down rate to see if it exceeds a threshold rate ofchange. If so, the heater 66 may be briefly activated in step 94. Thisactivation may continue until the cool down rate falls below thethreshold and the heater is deactivated in step 96.

As noted above, the particular threshold rate of change may varydepending on the characteristics of the scintillator crystal 36. By wayof example, if the scintillator crystal 36 is formed of LaBr₃, thedesignated maximum rate of temperature change may be 2° C. per minutefor a crystal with a diameter of about 2.5 in and a length of 3 in. Themaximum allowable rate of change is a function of crystal size, and maybe calculated through modeling techniques. Also, the amount of heatprovided by the heating device 66 may be calibrated to be sufficient toprevent the scintillator crystal 36 from cooling too rapidly, whilestill permitting the temperature of the scintillator crystal 36 tocontinue to drop.

While the embodiments set forth in the present disclosure may besusceptible to various modifications and alternative forms, specificembodiments have been shown by way of example in the drawings and havebeen described in detail herein. However, it should be understood thatthe disclosure is not intended to be limited to the particular formsdisclosed. The disclosure is to cover all modifications, equivalents,and alternatives falling within the spirit and scope of the disclosureas defined by the following appended claims.

What is claimed is:
 1. A scintillator comprising: a scintillatorcrystal; and a package at least partially surrounding the scintillatorcrystal, the package comprising a partially open Dewar flask that isopen to expose a face of the scintillator crystal to another componentthat extends beyond the partially open Dewar flask, wherein thepartially open Dewar flask is configured to allow the scintillatorcrystal to approach an ambient temperature of a well while preventingthe scintillator crystal from experiencing a rate of change intemperature sufficient to cause cracking or non-uniform light output, ora combination thereof.
 2. The scintillator of claim 1, wherein thescintillator crystal is configured to interface with a photomultiplierthat is at least partly outside of the partially open Dewar flask. 3.The scintillator of claim 1, wherein the partially open Dewar flaskcomprises an open aperture wider than a width the scintillator crystal.4. The scintillator of claim 1, wherein the partially open Dewar flaskis open to expose the face of the scintillator crystal to the othercomponent, wherein the other component comprises a light-transmissivewindow.
 5. The scintillator of claim 1, wherein the partially open Dewarflask is open to expose the face of the scintillator crystal to theother component, wherein the other component comprises alight-amplifying device outside of the partially open Dewar flask. 6.The scintillator of claim 1, wherein the partially open Dewar flask isconfigured to extend to cover part of a light-amplifying device coupledto the scintillator.
 7. The scintillator of claim 1, comprising anactive thermal protection element configured to actively prevent thescintillator crystal from experiencing the rate of change in temperaturesufficient to cause cracking or non-uniform light output, or thecombination thereof.
 8. The scintillator of claim 7, wherein the activethermal protection element comprises a heating element configured tocontrol a rate of temperature change in the scintillator crystal.
 9. Thescintillator of claim 1, wherein the rate of change in temperature isproportional to less than 2° C. per minute.
 10. A downhole tool fordetecting radiation in a well comprising: a scintillation detectorcomprising a scintillator crystal partially surrounded by a partiallyopen Dewar flask that is open to expose a face of the scintillatorcrystal to another component that extends beyond the partially openDewar flask, wherein the partially open Dewar flask is configured toallow the scintillator crystal to approach an ambient temperature of awell while preventing the scintillator crystal from experiencing a rateof change in temperature sufficient to cause cracking or non-uniformlight output, or a combination thereof.
 11. The downhole tool of claim10, wherein the scintillator crystal comprises LaBr₃:Ce; LaCl₃:Ce; or aLa-halide; or any combination thereof.
 12. The downhole tool of claim10, wherein the scintillator crystal is non-hygroscopic.
 13. Thedownhole tool of claim 10, wherein the scintillator crystal comprisesLuAP:Ce; LuYAP:Ce: YAP:Ce; LuAG:Pr; or Lu₂Si₂O₇; or any combinationthereof.
 14. The downhole tool of claim 10, wherein the partially openDewar flask is configured to allow the scintillator crystal to avoidcracking when the downhole tool is exposed to temperature changes ofmore than 3° C. per minute.
 15. The downhole tool of claim 10, whereinthe partially open Dewar flask is configured to allow the scintillatorcrystal to avoid cracking when the downhole tool is exposed totemperature changes of more than 5° C. per minute.
 16. The downhole toolof claim 10, wherein the partially open Dewar flask is configured toallow the scintillator crystal to avoid cracking when the downhole toolis exposed to temperature changes of more than 10° C. per minute. 17.The downhole tool of claim 10, wherein the partially open Dewar flask isconfigured to allow the scintillator crystal to avoid cracking resistantwhen the downhole tool is exposed to temperature changes of more than20° C. per minute.
 18. A package for a scintillator crystal comprising:a reflective layer configured to partially surround and couple directlyto a scintillator crystal; a layer of thermally conductive materialconfigured to partially surround the scintillator crystal; and apartially open Dewar flask configured to partially surround thescintillator crystal and to couple directly to the layer of thermallyconductive material.
 19. The package of claim 18, wherein the partiallyopen Dewar flask is configured to allow the scintillator crystal toapproach an ambient temperature of a well while preventing thescintillator crystal from experiencing a rate of change in temperaturesufficient to cause cracking or non-uniform light output, or acombination thereof.
 20. The package of claim 18, comprising a layer ofshock absorbing material configured to partially surround thescintillator crystal, to couple directly to the reflective layer and tothe layer of thermally conductive material, and to enable thermalexpansion of the scintillator crystal.